use OC_JSON::checkLoggedIn() in all ajax files

apps/calendar/ajax/activation.php

apps/calendar/ajax/choosecalendar.php

apps/calendar/ajax/deletecalendar.php

apps/calendar/ajax/deleteevent.php

apps/calendar/ajax/editcalendar.php

apps/calendar/ajax/editevent.php

apps/calendar/ajax/editeventform.php

apps/calendar/ajax/importdialog.php

apps/calendar/ajax/newcalendar.php

apps/calendar/ajax/newevent.php

apps/calendar/ajax/neweventform.php